As a rule, known optical position measuring systems provide, in addition to the incremental signals with respect to the relative displacement of two elements, so-called reference pulse signals. At a defined relative position it is possible by these signals to establish an exact absolute relation of the elements which can be moved with respect to each other. To generate the reference pulse signals, fields with reference marks are arranged at one or several places on the scale. Among other elements, the scanning unit of the optical position measuring system includes at least one reference mark scanning field, by which an appropriate reference pulse signal can be generated at the relative position of the scale and the scanning unit to be detected.
There are various options regarding the arrangement of the reference marks on the scale, or respectively the associated scanning fields on the scanning unit.
For example, it is known from U.S. Pat. No. 4,263,506 to arrange the reference marks laterally on the scale next to the graduation track with the incremental graduation. However, a problem here is that, if the scale and the scanning unit are twisted on an axis vertically with respect to the plane of the scale or scanning, the exact association of the reference pulse signal to a defined period of the incremental signal is possibly no longer assured.
Besides this it is also possible to integrate the reference marks directly into the graduation track with the incremental graduation, such as is suggested in U.S. Pat. No. 3,985,448. To this end it is possible, for example, to omit one or several strips or lines of the incremental graduation at the desired position of the scale in the incremental graduation. Further variations regarding the integration of reference marks into the track with the incremental graduation are known from German Patent Publication DE 35 36 466 A1, or from U.S. Pat. No. 4,866,269. It is proposed in these references to design the reference marks in the incremental graduation as a periodic sequences of lines or strips, or to utilize areas with optical properties, which differ from the remainder of the incremental graduation, as reference marks.
However, it has been shown to be problematic in connection with the integration of reference marks into the incremental graduation that next to the actual reference position it is customary to superimpose a periodic signal resulting from the incremental graduation of the scale on the detected reference pulse signal. An exemplary representation of the signal progression in this area is shown in FIG. 8. As a result, the modulation of the reference pulse signal is greatly reduced, i.e., assured detection of the reference pulse signal at the reference position is correspondingly difficult.
Of course it should also be pointed out as a further requirement in connection with the integration of a reference mark into the incremental graduation, that the smallest possible interference with the incremental signals by the reference pulse signal should result.